1. Field of the Invention
The present invention relates to ink set for inkjet printers; in particular, the ink set comprising a deep color ink and a light color ink.
2. Description of the Prior Art
Inkjet printing technique has been developed for several years. It is inexpensive, quiet, and with printing quality of colorfulness. Moreover, the ink can adhere to different kinds of medium, such as normal plain paper, paper for special printing and slide for printing.
Inkjet printing is similar to conventional printing. The basic ink set is a combination of cyan, magenta, yellow, and black. By modification, the combination can only be three colors including cyan, magenta and yellow to achieve full color effect. Moreover, the color saturation and printing quality can be enhanced by adding colors of light cyan, light magenta, light yellow, gray, green, blue, orange, etc.
Inks used for inkjet printing can be classified as dye-based ink or pigment-based ink according to compositions. The composition of ink is related to the presentation of colors. The dye is easily dissolved in water and presents brighter color. However, it usually has poor light resistance. Because of its good water solubility, the dye has poor water resistance. When the image from dye-based ink contacts water, the dyed ink dissolves again which damages the printing quality. The pigment is insoluble in water and has better water resistance and light resistance, but its color saturation is relative poor. Because the pigment is in particle form, it is hard to be dispersed uniformly and becomes larger particle to damage inkjet heads of printers. Therefore, a good dispersion is an important characteristic of the pigment-based ink.
On the other hand, inks used for inkjet printing can be classified as deep ink or light ink according to lightness. For analyzing the lightness of the inks, generally, the inventors use the method as following:
1. Ink solution dilute to 200 ppm of colorant content
2. Sample holder is quartz based with 0.2 cm thickness (holder thickness not wall thickness)
3. Measure the lightness of diluted ink solution by calorimeter with predetermined light source.
The deeper color has the smaller lightness. For example: the lightness of the black ink is generally smaller than 30, the lightness of the cyan ink is generally in a range 30 to 55, the lightness of the magenta ink is generally in a range 45 to 60, the lightness of the yellow ink is generally higher than 70.
Besides lightness, water resistance and light resistance, the drying speed and ink bleeding also affect printing quality. Fast drying means that the drying time for the ink on the printing medium is short. The drying speed will affect the printing speed of the printer, because the second diagram cannot be printed until the first one is dry. Ink bleeding means the ink spreading out or infiltrating into another color. It is because that the ink cannot cohere efficiently and also because of the capillary effect by the medium fiber. Ink bleeding will seriously damage printing quality.
There are several methods to restrain or retard ink bleeding on the medium. One method is to accelerate the drying speed by adding additives with higher volatility into the ink, such as low carbon-numbers alcohols. The second method is to accelerate the ink penetration on the medium by controlling the physical characters of the ink. This method not only makes the ink dry fast, but also restrains bleeding. The third method is to produce a wall to prevent infiltrating from one color into another. The wall can be produced by chemical reactions of two colors which contacting each other. The other method to retard bleeding is to add an effective surfactant into the ink to decreases the surface tension resulting in fast dry and printing speed.
Surfactant is commonly classified as cationic, ionic, non-ionic and amphoteric. Non-ionic surfactant has both hydrophilic and lipophilic sides, and is dissolved easily. In order to estimate its hydrophilic and lipophilic characters of the non-ionic surfactant, the Hydrophilic Lipophilic Balance (HLB) value is used. However, the algorithm for HLB value has not yet unified. The following is a common formula to calculate HLB.[(molecular weight of the hydrophilic composition)/(total molecular weight)]×20  Formula 1: 
According to Formula 1, the HLB value is ranged from 0 to 20. The higher the HLB value, the stronger the hydrophile character, and vice versa. By adding the surfactant into the ink, solubility of water and oil can be controlled and the surface tension of the ink can be adjusted.
Moreover, in the common desktop inkjet printing system, the color inks is mostly dye-based inks for enhancing the color saturation of image, and making images lighter. The black dye-based ink becomes less popular and is substituted by pigment-based ink, because the images with pigment-based ink can last longer. Because of the difference between dye and pigment, printing qualities of different ink sets are different.
Therefore, it is needed to accelerate the printing speed and improve printing quality. How to make a better combination of the deep/light inks, dyed/pigmented based inks, dyed/dyed based inks, and pigmented/pigmented based inks for inkjet printing becomes an important issue.